does the domain name of the FM server resolve to an IP to the outside world?
I am unclear as to what you mean. Could you rephrase?
Thank you for your post.
Is your ISP blocking the emails? Or, are some people receiving the emails while others do not? If the latter, then that is an ISP issue on the receiving end. You may want to contact them with their rules. For example, they may not accept emails if there is no subject or "To" field. If you send only BCC, then that could be a problem. They may not accept emails with more than X number in the To field.
from the internet - the source IP has to match the domain name it resolves to.
ie: if from the internet, you do a nslookup on 255.186.253.2 and it comes back mail.server.com
but if 255.186.253.2 says it's name is filemaker.server.com, the mail looks like spam and will probably be filtered.